Chapter 15 - Human Physiology
Chapter 15 - Human Physiology BIOL 2213
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This 13 page Class Notes was uploaded by Celine Notetaker on Sunday February 28, 2016. The Class Notes belongs to BIOL 2213 at University of Arkansas taught by Dr. Hill in Fall 2014. Since its upload, it has received 104 views. For similar materials see Human Physiology in Biology at University of Arkansas.
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Date Created: 02/28/16
Chapter 15 – Digestive Physiology Functions of the Gastrointestinal Organs – The mouth begins digestion with chewing. Saliva is produced by the salivary glands which are located in the head. Saliva moistens and lubricates food, and also contains amylase, which digests polysaccharides. Saliva also dissolves food so that food can react with chemoreceptors to produce taste. The pharynx and esophagus transfer food to the stomach. The glands lining the stomach wall secrete hydrochloric acid and pepsin. The acid disrupts the connective tissue proteins, so that food can be broken down. Pepsin then digests the proteins. HCl also kills bacteria. The digestive actions of the stomach produce chyme, which contains molecular fragments of polysaccharides and proteins, droplets of fat, salt, water, and other ions. Absorption of molecules occurs in the small intestine. The products of digestion are transported across the epithelial cells to enter the blood or lymph. The small intestine is divided into the duodenum, jejunum, and the ileum. Other organs include: −¿ ¿ 1. Pancreas – has an exocrine function that secretes digestive enzymes and HCO 3 . −¿ HCO ¿ 3 neutralizes the acidity of the chyme so that the digestive enzymes can function in the small intestine. 2. Liver – The liver has many functions, but in this section only the exocrine function of −¿ ¿ secreting bile is important. Bile contains HCO 3 , and a group of substances −¿ ¿ collectively called bile salts. CO 3 neutralizes stomach acid, and bile salts solubilize lipids. The ability to solubilize makes it so that rate of fat absorption is quicker. 3. Gall Bladder – The gallbladder stores bile. The gallbladder is a small sac that is located under the liver. In addition, it concentrates organic molecules by absorbing salts and water. During a meal, the gallbladder wall contracts, causing the concentrated bile solution to be injected into the duodenum via the common bile duct. 4. Small Intestine – The motility of the small intestine has 3 functions: 1) mixing luminal contents with various secretions, 2) brings contents into contact with epithelial surface for absorption and 3) advances luminal contents toward large intestine. 5. Large Intestine – Only a small amount of undigested material goes to the small intestine. Contraction of the rectum and relaxation by the sphincter muscles expel the feces in a process called defecation. Structure of the Gastrointestinal Tract Wall – From the midesophagus to the anus, the wall of the gastrointestinal tract has the general structure illustrated in the following figure. Mucosa – Just below the epithelium is the lamina propria, a layer of loose connective tissue that contains small blood vessels that run to the epithelium. Underneath the lamina propria is the Muscularis mucosa, a thin layer of smooth muscle that is involved in the movement of villi. The combination of the epithelium, lamina propria, and Muscularis mucosa is called the mucosa. Submucosa – The submucosa is a second connectivetissue layer. This contains a network of neurons called the submucosal plexus (from the autonomic nervous system) and blood and lymphatic vessels. Muscularis Externa – This is a muscle/nerve layer underneath the submucosa. Contraction of these muscles is responsible for moving and mixing contents in the gastrointestinal tract. The muscularis externa has 2 muscle layers and 1 nerve layer. There is a thick layer of circular muscle, whose fibers surround the tube so that contraction produces a narrowing of the lumen. The outer muscle layer is the longitudinal muscle, whose contraction shortens the tube. The nerve layer is called the myenteric plexus (from the autonomic nervous system). Serosa – The outer surface of the tube is a thin layer of connectivetissue called the serosa. The serosa is connected to the abdominal wall. Villi – Villi are fingerlike projections that extend from the luminal surface into the lumen of the small intestine. The surface of each villus is covered with a layer of epithelial cells that form more small projections called microvilli. The microvilli is called a brush border. The center of each villus is occupied by both a single, blindended lymphatic vessel called a lacteal, and a capillary network. Most fat absorbed by the body enters the lacteal. Other absorbed nutrients enter the capillaries. The venous drain from the small intestine enters the hepatic portal vein, which takes blood to the liver. In the liver, the blood flows through a secondary capillary network before leaving the liver and returning to the heart. Carbohydrate Absorption and Digestion – 2/3 of ingested carbohydrates is starch and the remaining 1/3 is sucrose and lactose. The digestion of starch begins in the mouth by the enzyme amylase, but only accounts for a small fraction of digestion. Most of the digestion of starch is completed in the small intestine by pancreatic amylase. The final products of starch breakdown are maltose and branched glucose. These products, along with ingested sucrose and lactose are broken down into monosaccharides (glucose, fructose, and galactose) by enzymes located in the luminal membranes of the brush border. The monosaccharides are transported across the epithelial cells and into the blood. The following mechanisms are responsible for monosaccharide adsorption. 1. Fructose – enters epithelial cells by facilitated diffusion via a glucose transporter (GLUT). 2. Glucose and Galactose – enters epithelial cells through secondary active transport coupled to Na via the sodiumglucose cotransporter (SGLT). a. Monosaccharides then leave the epithelial cells via facilitated diffusion from GLUT proteins through the epithelial basolateral membrane. Protein Digestion and Adsorption – There are 2 protein sources in the lumen. There are ingested proteins and enzymes (proteins) secreted by the body. Proteins are broken down into protein fragments by pepsin in the stomach, and by trypsin and chymotrypsin in the small intestine. Trypsin and chymotrypsin are secreted by the pancreas into the small intestine. Next, the protein fragments are broken down into amino acids by carboxypeptidase (from pancreas) and by aminopeptidase (from SI epithelial exocrine cells). The free amino acids enter the epithelial cells + by secondary active transport coupled to Na Some. mes, short chains of 23 amino acids can be absorbed by secondary active transport coupled to the hydrogen ion gradient. The amino acids leave the epithelial cells by facilitated diffusion. As with carbohydrates, amino acids are mostly absorbed by the upper part of the small intestine. Fat Digestion and Adsorption – Triglyceride digestion occurs almost entirely in the small intestine. The pancreas secretes lipase, which cleaves the triglyceride into 2 free fatty acids and a monoglyceride. Fats aggregate into droplets in the upper portion of the stomach. Lipase is a watersoluble enzyme, so it only works at the surfaces of the droplets. This is a slow process, so emulsion is necessary. Emulsification of Fat – The emulsification of fat requires 1) mechanical disruption of the large lipid droplets into smaller droplets and 2) an emulsifying agent, which acts to prevent the smaller droplets from aggregating back into large droplets. Phospholipids and bile salts are good emulsifying agents. The emulsifying agents coat the surface of the small lipid droplet. However, this coating prevents the action of lipase. To solve this problem, the pancreas secretes colipase, which is amphipathic and lodges on the lipid droplet surface. Colipase binds to lipase, and holds it on the surface. Action of Bile Salts in Fat Digestion – Still, the rate of fat digestion is slow after the processes described above. Bile salts form micelles, clusters with polar ends oriented toward the micelle’s surface and the nonpolar portions forming the inner core. Micelles contain the products of fat digestion (described above) and are in equilibrium with the small concentration of fat digestion products that are free in solution. Micelles are continuously breaking down and reforming. The steps of fat digestion are outlined below: 1. Triglycerides are broken down into monoglyceride and fatty acids by emulsification and lipase 2. Monoglycerides and fatty acids are encapsulated by micelles. 3. Micelles break down, allowing monoglyceride and fatty acids to diffuse across the epithelial plasma membrane. 4. Free lipids decrease, causing more micelles to break down. 5. While in the epithelial cell, monoglycerides and fatty acids are reformed into triglycerides by the smooth ER. This maintains a diffuse gradient so that monoglycerides and fatty acids can diffuse into the cell from the micelle breakdown. 6. Amphipathic proteins coat the surface of the newly formed fat droplet. 7. The smooth ER makes a vesicle around the fatprotein complex, sends it through the Golgi apparatus and the vesicle then fuses with the plasma membrane and is released through exocytosis into the interstitial fluid. 8. These extracellular fat droplets are called chylomicrons. 9. Chylomicrons pass into the lacteals via slit pores between their endothelial cells. The lymph from the small intestine eventually into the systemic veins via the thoracic duct. Vitamin Digestion and Absorption – Fat soluble vitamins (A, D, E, and K) follow the same pathway for fat absorption described above. Watersoluble vitamins are absorbed by diffusion or mediated transport. The one exception is vitamin B bec12se it is too large to diffuse properly. B 12ust first bind to a protein known as an intrinsic factor, which is secreted by the acid secreting cells of the stomach. The intrinsic factorB 12mplex binds to specific sites on the epithelial cells in the ileum. Vitamin B i12 hen absorbed by endocytosis. Water and Mineral Digestion and Adsorption – Water and minerals are absorbed through diffusion in the small intestine. Basic Principles of Gastrointestinal Processes – Gastrointestinal reflexes are initiated by a small number of luminal stimuli described below. The stimuli act on mechanoreceptors, osmoreceptors, and chemoreceptors located in the wall of the tract. 1. Distension of the wall by the volume of the luminal contents 2. Chyme osmolarity (total solute concentration) 3. Chyme acidity 4. Chyme concentration of specific digestion products like monosaccharides, fatty acids, peptides, and amino acids. Neural Regulation – The gastrointestinal tract has its own local nervous system called the enteric nervous system. There are 2 nerve networks, the myenteric plexus and the submucosal plexus. In general, the myenteric plexus influences smooth muscle activity whereas the submucosal plexus influences secretory activity. There are 2 types of reflexes: 1. Short Reflexes – from receptors through the nerve plexuses to effector cells 2. Long Reflexes – from receptors in the tract to the CNS by way of afferent nerves, and back to the nerve plexuses and effector cells by way of autonomic nerve fibers Hormonal Regulation – The hormones that control the gastrointestinal tract are secreted by endocrine cells scattered throughout the epithelium of the stomach and small intestine. Chemicals in the chyme stimulate the endocrine cells to release hormones. Gastrointestinal hormones reach their target cells via the circulation. Before you read further, potentiation is the phenomenon of 2 hormones working together to create a larger stimulus than either one working alone. There are 6 major hormones of the gastrointestinal tract: 1. Secretin – This is produced by the small intestine. It is stimulated by acid in the small intestine. It inhibits stomach acid secretion and inhibits motility. It stimulates bicarbonate secretion by the pancreas and liver. It potentiates the actions of CCK to stimulate release of enzymes from the pancreas. It works with CCK. 2. Cholecystokinin (CCK) – This is produced by the small intestine. It is stimulated by amino acids and fatty acids in the small intestine. It causes inhibition of stomach acid secretion and inhibits motility. It also causes enzyme secretion from the pancreas and gall bladder secretion of bile. It potentiates the action of secretin to stimulate bicarbonate release from gall bladder and pancreas. It works with secretin. 3. Gastrin – This is produced in the antrum of the stomach. It is stimulated by amino acids or peptides in the stomach. It causes stomach acid secretion and motility of the stomach, ileum, and large intestine. Stomach acid inhibits its release. 4. GlucoseDependent Insulinotropic Hormone (GIP) – It is produced by the small intestine. It is stimulated by glucose and fat in the small intestine. It stimulates insulin secretion. 5. Leptin – This is released by adipose cells and influences food intake and metabolic rate. Leptin induces satiety. 6. Ghrelin – This is released from the stomach during fasting. Ghrelin stimulates hunger. Phases of Gastrointestinal Control – There are 3 phases of neural and hormonal control of the gastrointestinal tract. The phases are differentiated according to where the stimulus is perceived. The 3 phases are the cephalic phase, gastric phase, and intestinal phase. 1. Cephalic Phase – initiated when receptors in the head are stimulated by sight, smell, taste, and chewing. Efferent neurons are parasympathetic fibers in the vagus nerve that activate neurons in the gastrointestinal nerve plexus, which affects secretory and contractile activity. 2. Gastric Phase – There are 4 stimuli in the stomach that are part of the gastric phase. These are distension, acidity, amino acids, and peptides (formed during the digestion of ingested protein). The end effect is the release of the hormone, gastrin from short or long neural reflexes. 3. Intestinal Phase – This is stimulated in the intestinal tract by distension, acidity, osmolarity, and various digestive products. Short or long neural reflexes cause the secretion of CCK and GIP. Swallowing Reflex – Pressure receptors in the walls of the pharynx are stimulated by the presence of food or drink. Receptors send afferent impulses to the swallowing center in the medulla oblongata. The swallowing center elicits swallowing via efferent fibers to the muscles in the pharynx, esophagus, and respiratory muscles. The soft palate elevates and lodges against the back wall of pharynx to prevent food from entering the nasal cavity. In addition, respiration is inhibited, raise the larynx, and close the glottis to keep food from moving into the trachea. The glottis is closed by the epiglottis, a flap of tissue that covers the glottis. Role of the Esophagus in Swallowing – Skeletal muscle surrounds the upper 1/3 of the esophagus and smooth muscle surrounds the lower 2/3 of the esophagus. Both ends of the esophagus are normally closed by the contraction of sphincter muscles. These 2 muscles are upper esophageal sphincter and the lower esophageal sphincter. Peristalsis is the process of moving food toward the stomach by a progressive wave of muscle contractions. These waves are called peristaltic waves. Stomach – The epithelial lining of the stomach invaginates the mucosa, forming glands. The glands in the upper portion of the stomach secrete HCl, mucus, and pepsinogen, the precursor to pepsin. The uppermost part of the stomach is called the fundus and lower part is called the antrum. The glands in the antrum do not secrete acid but instead secrete gastrin. Gastric Glands – Cells at the opening of the glands secrete mucus. There are 2 types of cells that line the walls of the glands. Parietal cells secrete acid and intrinsic factor. Chief cells secrete pepsinogen. Cells within the glands are called enteroendocrine cells, which secrete gastrin. In summary: 1. Mucus cells – secretes mucus 2. Parietal cells – secrete acid 3. Chief cells – secrete pepsinogen 4. Enteroendocrine cells – secrete gastrin Hydrochloric Acid Production – Primary H /K ATPase pumps H into the limen of the stomach. + + This also transports K into the parietal cell, which then leaks back into the lumen via open K channels (facilitated diffusion). On the other side of the parietal cell, bicarbonate (produced from carbon dioxide and water) is exchanged for chloride ion, which leaks into the stomach lumen via open chloride channels (facilitated diffusion). Regulation of HCl Production – Having more H /K ATPase in the cell increases acid secretion. + + The amount of H /K ATPase is regulated by 4 things described below. Parietal cells have receptors for all 4 of these molecules. The receptors initiate secondary messengers that are + + responsible for creating more H /K ATPase. 1. Gastrin – This is a gastric hormone that promotes acid secretion. 2. Acetylcholine (ACh) – This is a neurotransmitter that promotes acid secretion. 3. Histamine – This promotes acid secretion. 4. Somatostatin – This is a combination of 2 paracrine agents that inhibit acid secretion. Phase Regulation of HCl Production – In the cephalic phase (increase HCl), there is increased activity of parasympathetic nerves to the stomach’s enteric nervous system, resulting in the release of ACh from plexus neurons, gastrin from gastrinreleasing cells, and histamine from ECL cells. In the gastric phase (increase HCl), distention and the presence of peptides and amino acids causes long and short neural reflexes to produce gastrin. In the intestinal phase (decrease HCl), high acidity in the duodenum triggers reflexes that inhibit gastric acid secretion. Long and short neural reflexes produce secretin and CCK (enterogastrones), which inhibit the work of ACh, gastrin, and histamine. 1. Cephalic Phase – release of gastrin, ACh, and histamine 2. Gastric Phase – release of gastrin 3. Intestinal Phase – release of enterogastrones to inhibit release of gastrin, ACh, histamine Pepsin Secretion – Pepsin is secreted by chief cells in the form of an inactive precursor called pepsinogen. Low pH in the stomach causes pepsinogen to become pepsin by an autocatalytic process (acting on itself). Pepsinogen is a type of zymogen, which is an enzyme that is synthesized in an inactive form. Pepsin is inactivated in the small intestine because of the increased pH environment. Mechanism of Pepsinogen Secretion – Pepsinogen secretion parallels acid secretion, described above. It is based on processes that happen in the cephalic, gastric, and intestinal phases. The acid converts the pepsinogen to pepsin, which converts protein to smaller peptides. Gastric Motility – Stomach relaxation is due to parasympathetic nerves travelling to the stomach’s enteric nerve plexuses. The afferent signals travel up the vagus nerve and are processed by the swallowing center. The stomach produces peristaltic waves that travel from the body to the antrum. When the powerful wave reaches the antrum, the pyloric sphincter, a ring of smooth muscle and connective tissue, is closed. Therefore, only a small amount of chyme enters the small intestine at a time. The peristaltic waves are produced by pacemaker cells in the longitudinal smooth muscle layer. However, these waves would be small with no hormonal or neural input. All the factors that regulate acid secretion can also alter gastric motility. For example, gastrin increases contractions. Distention also increases the force of contraction. Gastric Emptying – Notice again that the inhibiting factors are the same factors that inhibit acid and pepsinogen secretion in the stomach. This prevents overfilling of the duodenum. The following chart illustrates how the duodenum decreases stomach emptying. Pancreatic Secretions – The pancreas has both endocrine and exocrine functions. The exocrine portion is responsible for secreting bicarbonate and a plethora of digestive enzymes. These products enter the pancreatic duct, which joins up with the common bile duct from the gall bladder, before it enters the duodenum. The following products are released from the pancreas: 1. Bicarbonate – This is produced in the same way that HCl is produced by parietal cells in the stomach. 2. Zymogens – These are inactive enzymes that digest proteins. They are inactive so that the pancreas (which has vital proteins in its cell membranes) are not digested. 3. Enterokinase This enzyme is responsible for converting inactive zymogens into their active form. They are embedded in the luminal plasma membrane of the intestinal epithelial cells. 4. Trypsinogen – This is the precursor to trypsin, which is a proteolytic enzyme (digests proteins). 5. Lipase – Digests fats 6. Amylase – Digests polysaccharides Control of Pancreatic Secretions – There are 2 main pancreatic secretions that must be controlled. The first is bicarbonate and the second are digestive enzymes. Bicarbonate is produced in response to high acidy in the duodenum. Digestive enzymes are produced in response to high intestinal fatty acids and amino acids. These phenomena are illustrated by the following mechanisms: The Liver – The functional units of the liver are called hepatic lobules. At the periphery of the hepatic lobules are the portal triads, which are composed of branches of the hepatic artery, hepatic portal vein, and the bile duct. The liver receives blood from 2 different blood vessels. First, the hepatic portal vein drains into the liver from the small intestine, so that absorbed nutrients can be processed before going to the rest of the circulation. Second, the liver is perfused by hepatic arteries that carry oxygenated blood to the liver. Both of these blood sourcs empty into a central vein (in the middle of the hepatic lubules) which feed into the hepatic veins that drain into the vena cava. Functions of the Liver – Liver cells, hepatocytes, secrete bile. The most important component of + bile are bile salts. During a fatty meal, bile salts are absorbed by Na coupled transporters in the ileum. The absorbed bile salts are returned to the liver via the portal vein, where they are once again secreted into bile. This recycling pathway is called the enterohepatic circulation. Any new bile salts that are needed are synthesized from cholesterol. The second main function of the liver is to break down old erythrocytes. The breakdown causes the release of bilirubin, one type of bile pigment. Bilirubin is extracted from the blood and secreted into the bile, which gives bile a yellow color. A third function of the liver is the synthesis of plasma proteins. Hepatic Portal System – Nutrients are absorbed from the small intestine into mesenteric veins which carry the nutrients to the liver via the hapatic portal vein. The hepatic portal veins branches into the hepatic portal system, which is a special vasculature that is responsible for processing nutrients. The hepatic portal system drains into the central vein. Bile Secretion and Liver Function – Fatty acids in the small intestine cause an increase in CCK, which causes more bile to flow into the common bile duct which increases bile flow into the duodenum. Liver Functions Summary – The liver has all the following functions: 1. Produce bile and bile salts 2. Produce bicarbonate 3. Produce hormones (angiotensinogen) 4. Produces plasma proteins (albumin and globulin) 5. Organic metabolism – glucose to glycogen 6. Cholesterol metabolum 7. Recycles red blood cell materials 8. Excretes foreign organic particles Secretion of the Small Intestine – The small intestine produces 1500 mL of fluid each day day. The reason is that the intestinal epithelium at the base of the villi secrete mineral ions (especially chloride ions) and water follows by osmosis. This fluid lubricates the surface of the intestinal tract and protects the cells from digestive enzyme damage. This water is reabsorbed and recycled. Motility of the Small Intestine – There is no peristalsis in the small intestine. Instead, there are stationary contractions and relaxations of individual segements. The rythmic contraction is celled segmentation, and produces a division of intestinal contents. Segmentation is initiated by pacemaker cells. The intensity of segmentation is controlled by hormones, enteric nervous activity, and autonomic nerves (parasympathetic division). 1. Gastroileal Reflex – This is when the intensity of the segmentation of ileum increases during periods of gastric emptying. This phenomenon shows that contractile activity in various regions in the small intestine by be altered by relfexes at different points along the gastrointestinal tract. Large Intestine – The first portion of the large intestine is the cecum. The sphincter between the cecum and the ileum is called the ileocecal valve, which is smooth muscle innervated by sympathetic nerves. This valve prevents backflow. The gastroileal reflex is responsible for opening the ileocecal sphincter, which is caused by segmentation. Basically, when you poop, more stuff can enter the large intestine from the small intestine. Most food is absorbed before entering the large intestine. Water is mostly absorbed in the large intestine. This is accomplished by actively transporting sodium out of the lumen so that water follows. Motility in the Large Intestine – Segmentation occurs in the large intestine. Materials move very slowly and remain in the large intestine for 1824 hours. 34 times a day, the large intestine produces a wave of intense contraction, known as mass movement. This mass movement spreads rapidly over the transverse segment of the large intestine toward the rectum. Parasympathetic input increases segmental contractions and sympathetic input decreases segmental contractions. Defecation Reflex – The anus is normally closed by the internal anal sphincter which is composed of smooth muscle, and the external anal spincter which is composed of skeletal muscle. The sudden distension of the walls of the rectum produced by the mass movement of fecal material into it initiates the neurally mediated defecation reflex.
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